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From: John Larkin on 20 Feb 2010 00:41
On Fri, 19 Feb 2010 23:26:38 -0600, Vladimir Vassilevsky
<nospam(a)nowhere.com> wrote:

>
>
>John Larkin wrote:
>> On Fri, 19 Feb 2010 20:38:12 -0600, Vladimir Vassilevsky
>> <nospam(a)nowhere.com> wrote:
>>
>>
>>>Is there such DAC in the world, that would allow for:
>>>
>>>* pure THD ~ 0.0001%, SNR is not very critical.
>>>* frequency response up to several kHz
>>>* DC accuracy in 10mV range
>>>
>>>
>> Here's 4 PPM
>>
>> http://www.ti.com/lit/gpn/dsd1792a
>>
>> in a "24 bit" dac. You could do some software predistortion to get
>> that down, conditions permitting.
>>
>> There are some claimed "32 bit" audio dacs!
>
>I have thought of audio DACs. Unfortunately, they have abysmal
>performance as far as zero shift and gain accuracy. I will probably have
>to calibrate those parameters at every measurement; that will slow down
>the things considerably. As for THD, a good analog filter is would be
>better for this application. The 32-bit interfaces to audio DACs/ADCs
>are quite usual, leaving the 32-bit performance claims on conscience of
>marketing.
>
>Tandeming two DACs (one for DC, the other one for AC) could be an
>option, however it looks like a heavyweight solution; I would rather do
>everything in one DAC.
>
>
>Vladimir Vassilevsky
>DSP and Mixed Signal Design Consultant
>http://www.abvolt.com


I use 16 and 20-bit audio-type DACs in NMR gradient drivers, where a
few PPM of zero offset gets attention. They seem to be pretty good at
DC, except that they don't have super-good internal references. I fix
that by ovenizing them. You just have to experiment when the parts
aren't spec'd for DC performance.

But how can you measure 1 PPM distortion? And where does it matter?

John



From: Ban on 20 Feb 2010 00:44
Vladimir Vassilevsky wrote:
> John Larkin wrote:
>> On Fri, 19 Feb 2010 20:38:12 -0600, Vladimir Vassilevsky
>> <nospam(a)nowhere.com> wrote:
>>
>>
>>> Is there such DAC in the world, that would allow for:
>>>
>>> * pure THD ~ 0.0001%, SNR is not very critical.
>>> * frequency response up to several kHz
>>> * DC accuracy in 10mV range
>>>
>>>
>> Here's 4 PPM
>>
>> http://www.ti.com/lit/gpn/dsd1792a
>>
>> in a "24 bit" dac. You could do some software predistortion to get
>> that down, conditions permitting.
>>
>> There are some claimed "32 bit" audio dacs!
>
> I have thought of audio DACs. Unfortunately, they have abysmal
> performance as far as zero shift and gain accuracy. I will probably
> have to calibrate those parameters at every measurement; that will
> slow down the things considerably. As for THD, a good analog filter
> is would be better for this application. The 32-bit interfaces to
> audio DACs/ADCs are quite usual, leaving the 32-bit performance
> claims on conscience of marketing.
>
if you think an active analog filter will give you -120dB THD performance,
go on dreaming. Some fixed frequency LC filter maybe.

> Tandeming two DACs (one for DC, the other one for AC) could be an
> option, however it looks like a heavyweight solution; I would rather
> do everything in one DAC.
>
I have seen better posts of yours.
ciao Ban


From: Phil Allison on 20 Feb 2010 01:02

"John Larkin"

> But how can you measure 1 PPM distortion?

** PC based audio measurement systems that use 24 bit sampling do it
routinely.

Just a matter of having a good FFT in software to resolve down to -130 dB.



> And where does it matter?


** It only matters to fuckwit TROLLS who dream up crazy specs while publicly
masturbating.



..... Phil



From: Vladimir Vassilevsky on 20 Feb 2010 01:19


John Larkin wrote:

> On Fri, 19 Feb 2010 23:26:38 -0600, Vladimir Vassilevsky
> <nospam(a)nowhere.com> wrote:
>
>
>>
>>John Larkin wrote:
>>
>>>On Fri, 19 Feb 2010 20:38:12 -0600, Vladimir Vassilevsky
>>><nospam(a)nowhere.com> wrote:
>>>
>>>
>>>
>>>>Is there such DAC in the world, that would allow for:
>>>>
>>>>* pure THD ~ 0.0001%, SNR is not very critical.
>>>>* frequency response up to several kHz
>>>>* DC accuracy in 10mV range
>>>>
>>>>
>>>
>>>Here's 4 PPM
>>>
>>>http://www.ti.com/lit/gpn/dsd1792a
>>>
>>>in a "24 bit" dac. You could do some software predistortion to get
>>>that down, conditions permitting.
>>>
>>>There are some claimed "32 bit" audio dacs!
>>
>>I have thought of audio DACs. Unfortunately, they have abysmal
>>performance as far as zero shift and gain accuracy. I will probably have
>>to calibrate those parameters at every measurement; that will slow down
>>the things considerably. As for THD, a good analog filter is would be
>>better for this application. The 32-bit interfaces to audio DACs/ADCs
>>are quite usual, leaving the 32-bit performance claims on conscience of
>>marketing.
>>
>>Tandeming two DACs (one for DC, the other one for AC) could be an
>>option, however it looks like a heavyweight solution; I would rather do
>>everything in one DAC.
>>
> I use 16 and 20-bit audio-type DACs in NMR gradient drivers, where a
> few PPM of zero offset gets attention. They seem to be pretty good at
> DC, except that they don't have super-good internal references. I fix
> that by ovenizing them. You just have to experiment when the parts
> aren't spec'd for DC performance.


> But how can you measure 1 PPM distortion? And where does it matter?

State of the art geophysic instruments specmanship is ~130+ dB of SNR
and ~120+ dB of THD (in the bandwidth of few hundred Hz). That's true
numbers; we have them already. However I need to do a self test circuit
that could confirm the performance numbers as well as a bunch of other
parameters. It doesnt't have to be super accurate; it has to be simple.
Board space is a premium, too.


VLV
From: Phil Allison on 20 Feb 2010 02:04

"Vladimir Vassilevsky = Ruskie TROLL "


> State of the art geophysic instruments

*** specmanship ***

> is ~130+ dB of SNR and ~120+ dB of THD

(in the bandwidth of few hundred Hz).

** He said it, not me.

Of course there are no transducers with THD anywhere near -120 dB.


That's true numbers; we have them already.


** But all SPECSMANSHIP !!!!!!!!!!

Equates to pure WANK VALUE.



..... Phil



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